1. Field of the Invention
This invention relates to hydrotreating hydrocarbon feedstock, and more particularly to regenerating hydrotreating catalyst.
2. Description of the Prior Art
In a hydrocarbon conversion system, such as for converting natural gas or fuel oil to hydrogen for use in a fuel cell or elsewhere, any sulfur in the hydrocarbon feedstock must usually be reduced to extremely low levels. For example, in fuel cell power plants wherein a hydrocarbon fuel is steam reformed to produce hydrogen for the fuel cells, economic considerations relative to the life and performance of the steam reform reactor catalyst make it desirable to reduce the sulfur content of the fuel fed into the reactor to less than one part per million.
In a typical system a steam reforming reactor is preceded upstream by a hydrodesulfurizer (HDS). The sulfur bearing hydrocarbon fuel and a small amount of hydrogen (usually bled from a point downstream of the reform reactor) is introduced into the HDS. In the HDS organic sulfur in the fuel combines with the hydrogen in the presence of, for example, sulfided nickel and sulfided molybdenum catalyst to form H.sub.2 S plus a harmless hydrogenated organic analog in accordance with the following equation: EQU Organic sulfur compounds+H.sub.2 .sup.cat. H.sub.2 S+organic analog
The effluent from the HDS is then introduced into hydrogen sulfide removal means, such as a bed of zinc oxide and/or charcoal which absorbs the hydrogen sulfide. The cleaned fuel may now be used in the reactor. Commonly owned U.S. Pat. Nos. 3,476,534 Buswell et al and U.S. Pat. No. 3,480,417 Setzer describe other fuel cell systems with sulfur removal means.
It is well known that hydrotreating catalysts, such as the sulfided nickel and sulfided molybdenum catalyst of the HDS discussed above, become deactivated with time by the accumulation of carbonaceous deposits. In the prior art it is common practice to regenerate the catalyst after it has accumulated carbon deposits amounting to several percent by weight of the catalyst. Known regeneration processes all have one important feature in common: regeneration is accomplished off line. Regenerating off line is expensive, but according to the prior art it has apparently been unavoidable.
In U.S. Pat. No. 4,033,898 a hydrocarbon conversion system is shut down (i.e., off line) while the hydrocarbon conversion catalysts are regenerated in situ. This involves purging the system of volatile-hydrocarbons using an inert gas, and then passing a gas containing from about 0.1 to about 2.0 volume percent oxygen through the bed of catalyst while maintaining the temperature between about 500.degree. F. and 1000.degree. F. for a sufficient time to burn off the carbonaceous deposits to the extent of reducing the carbon content of the catalyst to about 0.1 weight percent or less. The system is then again purged with an inert gas before putting the catalyst back on line. Other regeneration methods practiced in situ but during reactor shutdown are described in U.S. Pat. Nos. 2,934,493 and 3,966,587.
U.S. Pat. Nos. 4,026,821 and 4,038,209 describe a method and apparatus for regenerating nickel and molybdenum catalysts supported on alumina. The spent catalyst which has been contaminated with carbonaceous deposits as the result of being used in a refining process is removed from the refining apparatus, wetted with water, and then contacted with a free oxygen containing gas at a temperature in the range of 300.degree. F. to 600.degree. F. for a time sufficient to evaporate substantially all of the water from the catalyst surface before contacting the catalyst with a free oxygen-containing gas at a sufficiently elevated temperature to cause combustion of the carbonaceous deposits on the catalyst surface. The oxygen content of the regenerating gas may vary from below one volume percent to above twenty percent. Other regeneration techniques which use regenerators separate from the primary reaction apparatus are described in U.S. Pat. Nos. 3,041,290; 3,219,587; and 3,235,511.